The present invention relates to a vehicular automatic transmission for changing power transmission paths by controlling operation of shift means.
Automatic transmissions are arranged to shift gears automatically dependent on running conditions of a motor vehicle to achieve desired vehicle running characteristics. It is customary to provide a shift map composed of upshifting and downshifting lines for each speed range, the lines being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the gears according to the shift map dependent on the running conditions as indicated on the shift map. One example of such shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354, for example.
One type of automatic transmission includes a power transmission means comprising a plurality of power transmission paths (e.g., Plurality of gear trains), a plurality of shift means (e.g., a plurality of hydraulically operated clutches) for selecting the power transmission paths, and a control means (e.g., a hydraulic control valve) for controlling operation of the shift means. When a running condition of a motor vehicle, as indicated on a shift map, moves across an upshifting or downshifting line, a shift command is produced to effect an upshift or downshift, and a solenoid valve is operated based on the shift command to control operation of the hydraulic control valve to engage one of the hydraulically operated clutches. The power transmission path through a certain gear train associated with the engaged clutch is now selected to effect a shift.
If the solenoid valve is operated, at the same time as when the shift command is applied, to enable the hydraulic control valve (the control means) to operate the hydraulically operated clutch (shift means), then the shift can be performed without any time lag, but the driver of the motor vehicle may feel that the transmission operates too busily.
More specifically, it is assumed here that downshifts should be effected from 4th to 3rd to 2nd speed ranges by depressing an accelerator pedal. If the accelerator pedal is slowly depressed, then the driver feels that the transmission operates normally when the downshifts are performed in the order described above. However, if the accelerator pedal is quickly depressed to achieve a kickdown, for example, two downshifts are carried out in a short period of time when they are performed in the above sequence, and the driver feels that the operation of the transmission is too busy. The latter shift should preferably be effected by downshifting from the 4th speed range directly into the 2nd speed range while skipping the 3rd speed range.
To meet the above requirement, it has heretofore been customary to provide a decision timer for inhibiting a shift signal (e.g., a signal for operating the solenoid valve) within a preset period of time after a shift command had been issued. When a shift command for a downshift into the 3rd speed range while the motor vehicle is running with the transmission in the 4th speed range, and if a shift command for the 2nd speed range follows within the preset period of time, then the last shift command (a command for the 2nd speed range) is accepted while overriding the previous shift command. Upon elapse of the preset period of time, a shift is effected from the 4th speed range to the 2nd speed range, thus preventing the transmission from effecting successive shifts too quickly.
The above shift operation will be described in greater detail with reference to FIG. 8 of the accompanying drawings. In the graph of FIG. 8, while a motor vehicle is running with the transmission in the 4th speed range, a first shift command for the 3rd speed range is issued at a time t.sub.1, and then a second shift command for the 2nd speed range is issued at a time t.sub.2. At the time t.sub.1, a decision timer is operated, and an output from a shift solenoid is inhibited for a period of time T.sub.1 from the time t.sub.1 to a time t.sub.3. Since the second shift command is issued in the period of time T.sub.1, a shift output for the 2nd speed range is issued from the shift solenoid at the time t.sub.3 when the time period T.sub.1 elapses, thus effecting a shift from the 4th speed range to the 2nd speed range. Therefore, the hydraulic pressure in the 4th-sheed-range clutch is quickly lowered, whereas the hydraulic pressure in the 2nd-speed-range clutch is increased, at the time t.sub.3. However, since the 2nd-speed-range clutch starts being engaged a certain time lag after the hydraulic pressure in the 2nd-speed-range clutch starts rising at the time t.sub.3, the input and output rotational speed ratio e.sub.CLa (=the ratio between the rotational speeds of input and output shafts) of a target-speed-range clutch starts to change at a time t.sub.4. The rotational speed ratio e.sub.CLa remains 1.0 till the time t.sub.3 because any shift is inhibited till that time and a target speed-range up to that time is the present speed range (4th speed range in this case).
As described above, use of the decision timer is effective in preventing the transmission from effecting successive shifts too quickly. For reliable prevention of quick successive shifts, the period of time to which the descision timer is set should be of a certain long duration. While the decision timer is operating, however, no shift is carried out even if a shift command is issued, and there is a certain time lag before the target-speed-range clutch starts being actually engaged after the elapse of the present period of time of the decision timer. Consequently, if the present period of time of the decision timer is long, the time lag before the shift is performed tends to be too large.
For example, although the shift is issued at the time t.sub.1, the target-speed-range clutch (i.e., the 2nd-speed-range clutch) starts to be actually engaged only when the time time t.sub.4 is reached. Accordingly, the interval from the time t.sub.1 to the time t.sub.4 is present as a time lag, the interval from the time t.sub.1 to the time t.sub.3 being the preset period of time of the decision timer, and the interval from the time t.sub.3 to the time t.sub.4 being the period of time before the 2nd-speed-range clutch starts being engaged.
Even if the above time lag is present, the automatic transmission does not give the driver an odd feeling due to the time lag in normal operation because the driver cannot foresee when shifts are made. However, when the driver willfully starts a shift by quickly depressing the accelerator pedal or operating the shift lever or shift switch, if the above time lag is present, then the driver feels a delay in effecting the desired shift, and the driver will have an odd feeling with respect to the shift.